The present invention relates to a method and apparatus for machining an electrode workpiece by EDM by means of a travelling wire electrode continuously supplied between appropriate support and guiding members, the motion of one electrode relative to the other being controlled in such manner as to cut a predetermined path in the workpiece.
During such a machining operation, when the wire electrode cutting path in the workpiece is subjected to a sharp change of direction, there is generally a considerable error between the trajectory or path obtained in the workpiece and an ideal or programmed trajectory or path. The error is caused principally by the resultant of the forces acting on the wire electrode in the course of the machining operation, such resultant of the forces causing the active portions of the wire electrode to be no longer stretched as a straight line between its support and guiding members. The misalignment of the electrode wire relative to its support and guiding members may, for example, be of the order of 4/100 mm for a workpiece having a height of 70 mm. The method and apparatus disclosed in U.S. Pat. No. 4,081,652, assigned to the same assignee as the present application, permits to partially compensate for the error by varying the machining speed as a function of the curvature of the path. Such an arrangement, however, results in slowing down the machining speed when the path cut in the workpiece by the wire electrode has a substantial amount of curvature.
Another method, disclosed in Published German patent application No. 2,502,288, consists in passing through the wire electrode an auxiliary electrical current creating a magnetic field exerted in an opposite direction to the magnetic field resulting from the electrical discharges circulating through the wire electrode. Such a method cannot be used with wire electrodes of very small diameter because the density of the auxiliary current is too high to permit passing an effective machining current through the electrode wire.
The present invention permits to eliminate, for all practical purposes, the shortcomings and inconveniences of the methods and apparatus mentioned hereinbefore, and it permits to obtain a machining path exactly conforming to the machining path determined by a program, without decreasing the machining speed or the current supplied to the travelling wire electrode.